gnu-getopt: change keg_only reason as macOS has BSD getopt

[cho-m]

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On Linux, it usually makes more sense to use a general util-linux, which is how getopt is usually shipped in distros (rather than split packaged), e.g.

• Debian's util-linux - https://packages.debian.org/sid/amd64/util-linux/filelist
• Alpine's util-linux-misc - https://pkgs.alpinelinux.org/contents?branch=edge&name=util%2dlinux%2dmisc&arch=riscv64&repo=main
• Arch Linux's util-linux - https://archlinux.org/packages/core/x86_64/util-linux/
• Fedora's util-linux - https://packages.fedoraproject.org/pkgs/util-linux/util-linux/fedora-rawhide.html

[fxcoudert]

Makes sense to me, thanks!